The present invention relates to a dynamic pressure bearing apparatus and a method for manufacturing thereof and to a tool for forming a dynamic pressure generating groove in the apparatus and a method for manufacturing the groove, in which a dynamic pressure is generated in a lubricating fluid wherein a fixed member and a rotatable member hold each other relatively rotatable due to the dynamic pressure.
Various suggestions have been made regarding a dynamic pressure bearing apparatus holding various rotating bodies, such as a polygon mirror, a magnetic disc and an optical disc, while rotating at a high speed. In such a dynamic pressure bearing apparatus, a dynamic pressure bearing surface on the side of a fixed member is placed to face a dynamic pressure bearing surface on the side of a rotatable member with a narrow space or gap in which a dynamic pressure bearing portion is formed. Also, dynamic pressure generating grooves are formed on at least one of the dynamic pressure bearing surfaces wherein a lubricating fluid inserted in the dynamic pressure bearing portion, such as air and oil, is pressurized by a pumping effect generated by the dynamic pressure generating grooves during rotation such that the fixed member and the rotatable member are rotatably held thereat without contacting each other due to the dynamic pressure of the lubricating fluid.
The dynamic pressure bearing grooves are formed as a concavity extending on the dynamic pressure bearing surfaces in a shape of a spiral or a herringbone. A projecting portion, as the rest of the space on the dynamic pressure bearing surfaces, comprises a projecting surface. When the rotatable member is not in rotation, the fixed member and the rotatable member are in contact with each other wherein the lubricating fluid is held inside the dynamic pressure generating grooves. Once relative rotation of the rotatable member and the fixed member starts, the lubricating fluid in each of the dynamic pressure generating grooves flows towards a specific pressure point and generates a dynamic pressure by confluence thereof to provide a given floating power.
As described above, in an ordinary dynamic pressure bearing apparatus, it takes a period of time to obtain a given floating power by a dynamic pressure after rotation starts; in other words there is a period of time at the beginning of the rotation when there is no dynamic pressure in the lubricating fluid. Therefore, the fixed member and the rotatable member rub each other while being in contact during the above period of time. As a result, abrasion of the members progresses in the early stage of their life, resulting in life shorter than expected.
Such an issue becomes noticeable when original surface 2xe2x80x2 (indicated as a dotted line), as shown as a thrust dynamic pressure bearing surface in FIG. 17, is flattened to increase flatness of projecting portion 2, comprising a projecting surface as the remains area thereat other than dynamic pressure generating groove 1. In other words, projecting portion 2 after the flattening treatment has its surface slightly lower than original surface 2xe2x80x2; then, the opening edge of dynamic pressure generating groove 1 is pushed inward by the difference in the height. As a result, reverse tapering portion 1b starts from the bottom of the side wall of the groove. Such reverse tapering portion 1b tends to keep back the lubricating fluid which tries to flow out from dynamic pressure generating groove 1 towards projecting portion 2; especially, when rotation has just started, the time period without the lubricating fluid on the side of projecting portion 2 is further extended such that abrasion of the fixed member and the rotatable member is significantly increased.
Hence, a purpose of the present invention is to provide a dynamic pressure bearing apparatus, which has a simple configuration wherein abrasion of a fixed member and a rotatable member is decreased, and a method for manufacturing thereof and a tool for forming a dynamic pressure generating groove used therewith and a method for manufacturing thereof.
In brief, the dynamic pressure bearing has first and second facing bearing surfaces having a gap between them. Typically, one of these bearing surfaces might be the surface of a shaft of a motor and the other bearing surface might be the cylindrical bearing portion of the stator of the motor. As is known in the art, a plurality of pressure generating grooves are formed on one of the two bearing surfaces. These grooves contain lubricating fluid. When the rotor rotates, these grooves tend to pump fluid into the gap between the two bearing surfaces. Thus the fluid flows from the grooves to the space between the top of the projections that define the grooves and the opposed surface. It is the top of the projections which bear most of the load. In order to enhance the flow of the lubricating fluid into the zone adjacent to the top of the projections, a beveled surface is provided at each corner of the projections. This bevel provides a wedge-shaped space that enhances lubricating fluid flow from the grooves to the gap.